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Is Bunnings 42mm x 42mm Merbau suitable for joists for low level decking?

Hi all, I'm doing some low level decking and wanting to use the Bunnings 42mm x 42 mm (pencil rounded) Merbau timber for the joists. They will be on a concrete slab but I'm really limited to the height, hence the need for the 42mm x 42mm.

I propose Bunnings mini feet for the bearings.

I'll be applying Ekodeck boards (180mm width) on top with 6mm gaps. Area is 22m2.

Height, off the concrete, for the joists will range from 10 mm to 40 mm (Yes, very low level).

I'll be applying bearer protector (CA group) G-tape onto the tops of the joists.

The concrete has a drain in the middle with slope towards the drain from each side.

Most of the decking (say 2/3rds) is permanently in the dry, under cover. The exposed area cops the Perth weather.

The merbau timber is rated to Class 1. Initially, I was looking at a H4 rated softwood (Radiata Pine) for the joists (due to the low height off the concrete) but from what I can see, the span for them is 250mm and that would involve around 200 bearers.

Wondering what the span capability is for the Merbau hardwood and whether people more experienced than I am, would say it's up to the task given the low level.

Your experience and recommendations are welcome

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Comments

EricL

Hello @Henry1

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your low-level decking project.

I'm afraid the 42 x 42mm 2.7m Pre-Oiled Merbau Solid Screening will not be sufficient for your project. It is rated for fencing not for framing and will therefore not be able to carry heavy loads. A close alternative would be the 90 x 45mm Outdoor Framing H3 Treated Pine. Can you please tell us the maximum and minimum space in height you are working with?

If you were to use the 90x45 and the frame is built with the 45mm side up, you would be occupying a space of 90mm + 23mm which equals to 113mm and whatever is left underneath will be the air gap of your deck. If you were to build it with the 90mm side up the frame would not be as strong and you would need to use more support to hold the frame and decking panels in place.

Would it be possible for you to post a few photos of the proposed decking area? We might be able to offer recommendations on how to lay it out.

In the meantime, I suggest having a quick look at these guides - How to build a low-level deck by @Adam_W and How to build a deck by @MitchellMc.

Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.

If you need further assistance, please let us know.

Eric

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/56465#Comment_56465

Good Afternoon @Henry1

I am thinking of a few pics that showcase the area. The 42mm square merbau would not be suitable as @EricL has mentioned.

When you say 22sqm but later mentioon 200 bearers I am a bit confused.

Length = ?

Breadth = ?

Height from concrete to door sill level = ?

All help us out in picturing what you have.

Dave

Henry1

https://www.workshop.bunnings.com.au/discussion/comment/90513#Comment_90513

Screenshot (164).png

Henry1

https://www.workshop.bunnings.com.au/discussion/comment/90513#Comment_90513

Hi Dave

Photos just posted. I have a 100mm gap from the concrete base to the swing of the back security door. The concrete is curved so flow goes under the decking to the middle, to the drain (pictured). As a result, the outer area of the concrete - which cops the weather - is relatively higher and allows only about 10-15 mm for a bearer under the joist.

Also in the photos posted is a screenshot of H4 radiata pine I could possibly use for the joists, given the low level ? Using these though will mean more bearers. Any help on span recommendation would be welcome. From what I could glean from google was 250mm spacing meaning 4.8 m (Width) x 4 per metre x 10 joists = Up to 192 bearers (plus I'll need some for noggins).

Area is 4.8 m W X (Up to) 4.5 m L (end is rounded and gradually reduces to the minimum L 3.6m).

Joists will be at 450mm

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/111158#Comment_111158

Hi @Henry1,

Unfortunately, you'll notice in the Terraframe Install Guide & Span Tables that the 45x45mm pine doesn't have a listed span because it is not capable of supporting an elevated load.

In this situation, where you have only 100mm of height to work with, you would be better off ditching the idea of having both joists and bearers and treating every framing member as a joist, where it is directly supported from below and does not structurally support any other framing member.

If you opted for a joist-only deck frame, you could use the 70 x 45mm MGP10 H4 Treated Outdoor Structural Radiata Pine, which, according to the span tables I linked to above, can span 750mm as a continuous span, meaning it runs over multiple supports, not just two.

This means you would need to support each joist every 750mm with a Builders Edge 25-40mm Minifoot Pedestal Foot.

You would then add noggins and rim-joists to keep all your framing stiff and parallel. As these framing members would not directly support weight, they are not required to follow the span tables.

Can you let me know which way you want the decking boards to run in relation to the door? A quick top-down drawing with the measurements and oritentation of the decking boards would be great.

With this information, I would be happy to throw together a basic plan for your deck frame.

Let me know what you think.

Jacob

Henry1

https://www.workshop.bunnings.com.au/discussion/comment/122979#Comment_122979

Hi Jacob

Thanks for getting back to me.

I'm planning on using Ekodeck summit 180mm Width planks. (23mm H)

Basic plan is pictured. I was playing with putting a breaker board in (pictured), to reduce some wastage as the ekodeck are 5.4 m lengths.

I was thinking 1.8m Length from the house then a breaker board then completing the rest.

I will need some mini feet or spacers in particular in the middle as the concrete slopes in from the outside (think skate bowl) Screenshot 2025-08-29 075602.png

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/130722#Comment_130722

Hi @Henry1,

I've done up a quick framing plan using the 70 x 45mm MGP10 H4 Treated Outdoor Structural Radiata Pine.

With this orientation, each joist would need to be supported every 750mm, which, at this width, would require 8 supports per full-length joist.

The breaker board could be a good idea to reduce wastage, although you would need to add a second joist wherever it is situated so the board is supported across its full width. If the cost to add another joist, with 8 supports, outweighs the cost of the wastage, then it would be worth doing.

Let me know what you think.

Jacob

Henry1

https://www.workshop.bunnings.com.au/discussion/comment/135487#Comment_135487

Thanks again Jacob

I'm not sure I'll be able to accommodate the 70 mm joist height at the outer edges (opposite edges from the door). .

. The problem there is if I don't use spacers, the boards on the concrete will block drainage flow, leaving water to pool.

Next to the door is fine as it's well under cover.

I'll do some testing but I'm not confident there's enough height on the outer edges.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/138718#Comment_138718

Hi @Henry1,

Would you be willing to change the direction of the decking boards?

If so, then you could run your joists perpendicular to the house, which would allow any water to flow between them.

On the outer edges where there is minimal height, you could sit your joists on window packers, then bolt them to the concrete for stability using angle brackets.

Outside of this approach, unfortunately, I can't see any way of making it work besides cutting out the concrete to create more height for the frame.

Let me know what you think.

Jacob

Nailbag

https://www.workshop.bunnings.com.au/discussion/comment/138718#Comment_138718

Hi @Henry1

Runnings straight down to @JacobZ postings, I would suggest exactly the same utilising the H3 treated 75mm x 45mm as your joists and would consider his frame layout. However in dealing with your concern and conceding you have a centrally located drain, could you use a demolition saw to make cuts towards the centre. This combined with a pneumatic hammer to chisel out channels to the drain which toy could if possible remove and lower it to accomodate the channels. These wouldn't need to be very deep and perfectly made being unseen.

Nailbag

Henry1

https://www.workshop.bunnings.com.au/discussion/comment/140854#Comment_140854

Good thinking. I might give that a go. Thanks!

Henry1

https://www.workshop.bunnings.com.au/discussion/comment/140853#Comment_140853

Hi .@JacobZ

Water/weather ingress is on two sides, unfortunately at right angles. That's where all the rotted timber (boards and joists) were from the previous deck.

I was planning to use the window packers on the lower rise joists (and mini feet on the higher central points) as there's a heap that I've retrieved from the previous deck. But I might see if I can chisel out a drainage channel like Nailbag suggested (to allow for water to not pool). That might be workable.

Henry1

https://www.workshop.bunnings.com.au/discussion/comment/140853#Comment_140853

Mmmm, the slope on the two outer edges - which cop all the weather - are quite high insofar as the concrete is concerned. It's quite dished, like a ultra small skate park.

I'll be able to get away with 70 x 45mm on 70% of the joists but will need to use 45mm as a maximum elsewhere. The alternative is to have the boards slope upwards slightly, as they go out from the house.

I think I'm going to have to use the 40 x 50mm hardwood timber I salvaged from the original deck and place it on the two outer edges and run 5 to 10mm window packers under them. I know this isn't ideal and the downside is I won't get anywhere near as long from the subframe as ideal.

What's the best way to join 70mm H pine board with a 40mm H hardwood board? I'll need to do this on 7 of the joists.

Also, do I need to bolt the subframe down giving the weight once finished with the decking. And if so, what could be a minimum I could get away with?

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/142437#Comment_142437

Hi @Henry1,

There is no great way to attach your 70x45 to the 40mm hardwood because it's not how decks are typically built. I suppose the best you could do is rip the 70x45 down where needed and then screw it to the hardwood with long batten screws.

Yes, you should bolt the frame down. Regardless of the weight of the deck, if wind gets under it, it can cause uplift. I would be looking at using at least 3 angle brackets per joist.

Let me know if you have any further questions.

Jacob

Henry1

https://www.workshop.bunnings.com.au/discussion/comment/140854#Comment_140854

Hi Nailbag

I'm still working on this project and have bought 1 x H3 70 x 45mm board to test it in position for each joist. The first position closest to the door worked (just - with about 2mm spare). But wondering was there any reason (s) why you suggested the H3 rather than the H4 board? Notwithstanding the higher cost/ special order, might the H4 might be better for durability for this job given the ultra low setting?

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/142443#Comment_142443

Hi @Henry1,

Allow me to tag @Nailbag so he is notified of your question.

I'd say he just mentioned H3 as it is what has commonly been used for low-level decks. H4 is a better option in close proximity to the ground, especially considering the drainage issues you have already experienced.

Jacob

Nailbag

https://www.workshop.bunnings.com.au/discussion/comment/142443#Comment_142443

Hi @Henry1

Apologies for the typo confusion. Yes to H4 treated over H3 as @JacobZ correctly picked up as well.

Nailbag

Henry1

https://www.workshop.bunnings.com.au/discussion/comment/140853#Comment_140853

Hi .@JacobZ

I'm still progressing on this. I've laid out the 70 x 45 joists and most will need shims rather than the mini feet I was hoping to use. Can I nail (stainless steel) or screw the shims to the underside of the joists. I saw someone do it on a youtube but your comment above suggested angle bracket/ bolt to the concrete.

I could still do the bolt to the concrete - say 3 per joist - but was hoping for an easier way for the rest of the shims.

By the way, when I bolt the bracket to the concrete, would ankascrews be ok? After allowing for the shim(s), what length of the screw do you recommend being in the concrete?

And I assume the shim(s) are placed not under the actual joist but under the bracket like this diagram Screenshot 2025-09-17 063120.png Screenshot 2025-09-17 063223.png Screenshot 2025-09-17 063317.png

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/142457#Comment_142457

Hi @Henry1,

Yes, you could fix the shims to the bottom of the joists with fixings. Shims typically go under the joists, not the bracket. If there is a need to shim the bracket, it would be best to use larger brackets instead. Ankascrews would be suitable and you'd need at least 40mm of their length into the concrete, so the 50mm you've shown would be suitable dependant on the thickness of the shims, if you're using them under the bracket.

Mitchell

Henry1

https://www.workshop.bunnings.com.au/discussion/comment/143581#Comment_143581

Hi .@MitchellMc.@JacobZ and @Nailbag

Thank you all for your help. I finally finished the project.

Summit 'Fairhaven' boards with Classic greystone fascia boards.

I went with a breaker board for a couple of reasons. One was reduced wastage and cost, notwithstanding much more time on the job. I saved around $300 by being able to plan the best cuts to maximise the 5.4m lengths. The second reason was the previous deck substructure in the outer area was quite rotted. When that happens again, I might be able to just pull off the composite in the outer area, fix the substructure then put the composite back.

Some pics.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/144457#Comment_144457

That looks fantastic @Henry1 and looking at the support structure, I imagine it is dead sturdy.

The decision to use a breaker board was certainly worthwhile considering the cost reduction.

I can't say I'd seen the Fairhaven / Stirling Summit Series Composite Decking on a deck before now, but I am really loving the look. It looks lovely and bright and creates a really welcoming space.

I hope you are extremely proud of what you've achieved.

Thank you for sharing.

Jacob

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